A natural joint is a connection between two bones and is classified under two broad categories: (i) joints lacking a joint cavity which allow little or no movement, such as the joint between adjacent vertebrae; and (ii) joints having a joint cavity which allow free movement of such joints in the body. These joints are called synovial joints or synovial-type joints. In humans, common synovial joints include the hip joint, the knee joint, the ankle joint, the shoulder joint, the elbow joint, the wrist joint, the finger joint, the finger metacarpal joint, the toe joint, the toe-metatarsal joint and the carpometacarpal joint. Synovial-type joints can be further classified into three categories: uniaxial, biaxial and triaxial. Uniaxial joints can be further categorized into hinge and pivot joints. Examples of hinge joints are the joints of the fingers, known as the interphalangeal joints. Pivot joints are formed by a central bony pivot surrounded by an osteo-ligamentous ring. Movements are permitted in one plane around a vertical axis. Examples of this type of joint are the superior and inferior radioulnar joints (i.e., joints of the elbow) and the median atlantoaxial joint (i.e., upper neck joints). In a biaxial joint, motion occurs in two planes; thus permitting two degrees of freedom. There are two types of biaxial joints, saddle joints and condyloid joints. An example of a saddle joint is the carpometacarpal joint of the thumb, where the bones fit together resembling an individual riding a horse while sitting on a saddle, with one bone being concave and the other being convex. Examples of condyloid joints are the metacarpophalangeal joints of the fingers. Triaxial joints have three degrees of motion and permit movement in three planes. There are two types of triaxial joints, ball and socket joints, and plane joints. Examples of ball and socket joints include the shoulder joint and the hip joint.
FIGS. 1-4 illustrate artificial joint implants according to the prior art. These joint implants are presented herein in order to facilitate understanding of the field of the disclosed technique. Reference is made to FIG. 1, which is an exploded side perspective view of a total hip replacement implant, as is known in the prior art. Total hip replacement implant 10 includes a cup 11, a liner 12 and a stem 13. Stem 13 includes a head 14. Hip replacement implant 10 can be categorized as a ball-and-socket joint, i.e., one in which the rounded surface of head 14 fits into and moves within a cup-shaped depression of liner 12 which fits into cup 11. Such a ball-and-socket joint allows freedom of movement up, down, right, left and in a full 360-degrees of rotation.
Reference is now made to FIG. 2, which is an example of a total knee replacement implant, generally referenced 20, as is known in the prior art. The knee joint is a pivotal hinge joint which permits flexion and extension as well as a slight medial and lateral rotation. Total knee replacement implant includes a head 21, a liner 22 and a stem 23. Head 21 is coupled with liner 22 which is coupled with stem 23. Reference is now made to FIG. 3 which is a side perspective view of a total disc replacement implant, generally referenced 30, as is known in the prior art. Total disc replacement implant includes a top plate 31, a liner 32 and a base plate 33. Reference is now made to FIG. 4 which is a total ankle replacement implant, generally referenced 40, as is known in the prior art. Total ankle replacement implant 40 includes a top plate 41, a liner (or insert) 42 and a base plate 43.
Common materials which have been used over the years for medical joint implants are: metals, ceramics, polymers and composite materials. Different types of medical joint implants include metal-on-metal implants, which are made of stainless steel or cobalt chrome alloys, ceramic-on-ceramic implants, which are made of zirconia and alumina, polymer implants such as Ultra-High-Molecular-Weight Polyethylene (UHMWPE) and polyimides, and composite material implants such as carbon fiber-reinforced PEEK (polyether ether ketone).
Metal-on-metal joint implants as well as ceramic and UHMWPE joint implants each suffer from a common drawback, viz. mechanical wear and degradation of the implant in a patient's body due to the repetitious movement of one part of the implant in respect of the other part of the implant. For example, in 2010, there was a great deal of publicity in the USA on the failure and recall of the metal-on-metal hip marketed by DePuy. The recall came after data from a study indicated that the five year failure rate of this metal-on-metal hip is approximately 13%. Even if the defective device is replaced, it can leave behind dangerous, possibly deadly fragments that may not be discovered for years. DePuy identified reasons for the failure of the hip replacement system as component loosening, component malalignment, infection, fracture of the bone, dislocation, metal sensitivity and pain. In general, with respect to all metal-on-metal artificial joints, the movement of the metal head within the metal cup of the joint implant results in a high volume of metallic debris which is absorbed into the patient's body. The absorption of metallic debris by the body can cause inflammatory reactions, resulting in pain in the groin area, death of tissue in the hip joint and actual loss of surrounding bone. Polymer joint implants suffer from similar drawbacks with respect to degradation and wear debris. Every year, more than a million Americans receive an artificial hip or knee prosthesis. Such joint implants are designed to last years but in about 15-20 percent of patients who receive a total joint replacement, the joint implant prematurely loosens and has to be replaced early, which can cause dangerous complications in patients, in particular in elderly patients. Over the years the durability of artificial joint implants has improved, for example a hip implant now lasts between 10-15 years and a knee implant lasts between 7-10 years, yet the average age of patients in need of an artificial prosthesis has significantly lowered. Moreover, as active participation in sports, both recreational and professional, is an increasing reality for a significant number of younger patients, there is a growing need for a durable joint implant which exhibits a higher lasting rate.
As friction is a major cause of tear and wear of joint implants, attempts to provide durable joint implants aim at developing new ceramic or composite materials having a higher wear resistance and a lower friction coefficient. One attempted solution to overcome the accelerated wear of joint implants due to the continuous friction of non-lubricated surfaces of the joint implant was made by coating one or both of the contacting surfaces with films of hydroxyapatite, thus reducing the coefficient of friction between the contacting surfaces. Such films however end up being quite thick and unstable, and tend to break off from the joint implant.
Many examples of attempts to overcome the accelerated wear of joint implants are known. U.S. Pat. No. 5,976,190 to Klaus-Peter Anhalt et al., entitled “Orthopaedic connection” is directed to an orthopedic clamp connection as a functional element for force transmission in a prosthesis comprising a tube socket and a tube made of light metal, and a contact area there between. The contact area includes an intermediate layer comprising a low molecular weight carrier material in which at least one of a lubricant or an auxiliary is embedded. The carrier material is selected from inorganic materials such as metal, metal salt, metal oxide or ceramic material. The intermediate layer has a thickness from 0.1 μm to 1 mm. Suggested lubricants are a dicarboxylic ester, a fatty acid, a fatty acid ester, a fatty acid amide, a metal soap, a silicone oil, molybdenum sulfide, a polyolefin wax, a paraffin, a fluoropolymer or a combination thereof.
PCT International Application Publication No. WO 2009/115790 to Peter White, entitled “Replacement bone joint” is directed to a replacement bone joint that has joint members such as a femoral prosthesis and an acetabular prosthesis which define a ball and socket joint between a head and a titanium cup. The titanium cup has a lining and the head, which is made of stainless steel, is provided with an adhered coating. The coating and lining are formed from a non-crossed-linked thermoplastic material such as nylon, which may be incorporated with lubricants such as silicone, hydrogels, molybdenum sulfide, polyvinyl pyrollidone, glycerin, mineral oil grease and/or graphite.